Many and various circuits have been proposed to detect and indicate faults, and often such circuits monitor lighting or signalling circuits. Examples are to be found in the following U.S. Pat. Nos.:
3,143,729--Power PA0 3,626,401--Flieder PA0 3,644,886--Sabaroff PA0 3,710,157--Wright PA0 3,855,586--Jacobs PA0 4,259,659--Ariyoshi et al
However, the problem encountered in aircraft signalling light systems which are required on unusual obstructions, such as towers, chimneys and the like, is particularly troublesome because mere lamp failure is not the only serious fault which may be encountered. Rather, such systems usually employ top lights which flash intermittently. Obviously, a major fault can exist when one or more of such top lights fails and also when the flasher unit has failed either in the "on" or the "off" conditions. For the side lights, it is sufficient to know when at least one of them has failed.